Trichomoniasis is the most common nonviral sexually transmitted disease worldwide. It has been estimated that more than 180 million people are infected with Trichomonas vaginalis. Infection with T. vaginalis can be asymptomatic or can be associated with acute vaginitis. T. vaginalis infects the squamous epithelium of the lower genital tract almost exclusively, in both women and men. In women, the vagina, exocervix, urethra, and Skene’s glands are the main sites of infection. In men, the infection is urethral and usually without symptoms. Some investigators have found an association between T. vaginalis infection and prematurity. After successful therapy, recurrent vaginal infection may occur. However, T. vaginalis almost never spreads outside the human genitourinary system. Infection with T. vaginalis may elicit an inflammation characterized by discharge. Symptoms of vaginal trichomoniasis include abnormal frothy discharge and genital irritation. Observation of live protozoa in specimens from the vagina or urethra is definitive evidence of genital trichomoniasis. Trichomonas culture is the most sensitive diagnostic procedure but will miss up to 15% of infected cases. Monoclonal antibodies to T. vaginalis have been used to identify vaginal trichomoniasis. Metronidazole can produce chromosomal alterations in bacteria and may increase the incidence of tumors in susceptible animals. In summary, T. vaginalis is likely to be an important factor in prematurity. Better diagnostic tests (amenable to screening use), better therapy, and appropriate clinical trials are needed to determine whether screening and treatment should be used to prevent prematurity.